In general, automotive parts including a gear, a transmission unit and a seat recliner are manufactured from a hot-rolled steel sheet which is a carbon steel for machine structural use conforming to JIS G 4051, by forming the steel sheet into a specified shape by using cold forming and then giving the resulting work a specified hardness by performing a quenching treatment. Therefore, there is a demand for a hot-rolled steel sheet that has excellent cold formability and hardenability when in a raw material state. To meet this demand, Japanese Unexamined Patent Application No. 2002-309345 discloses a hot-rolled steel sheet having excellent impact strength after quenching treatment has been performed, which has a chemical composition containing, by mass %, C: 0.10% or more and 0.37% or less, Si: 1% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, sol.Al: 0.01% or more and 0.1% or less, N: 0.0005% or more and 0.0050% or less, Ti: 0.005% or more and 0.05% or less, and B: 0.0003% or more and 0.0050% or less, in which the relationship B−(10.8/14)N*≧0.0005% is satisfied, where N*=N−(14/48)Ti, and where N*=0 in the case where N−(14/48)Ti≦0, in which the mean particle diameter of TiN, which is a precipitate in steel, is 0.06 μm or more and 0.30 μm or less, and in which the mean grain diameter of prior austenite is 2 μm or more and 25 μm or less after quenching treatment has been performed. The method disclosed by JP '345 is described as being capable of providing the hot-rolled steel sheet described above by hot-rolling the steel having the chemical composition described above, in which the coiling temperature is 720° C. or lower.
Japanese Unexamined Patent Application Publication No. 5-98356 discloses a method for manufacturing a tempering-free-type high carbon thin steel sheet having a thickness of 4 mm or less and having a tensile strength (TS) and an elongation (El) satisfying the relationship TS×El≧16000 MPa % by cold-rolling steel having a chemical composition containing, by mass %, C: 0.15% or more and 0.40% or less, Si: 0.35% or less, Mn: 0.6% or more and 1.5% or less, P: 0.030% or less, S: 0.020% or less, Ti: 0.005% or more and 0.1% or less, sol.Al: 0.01% or more and 0.20% or less, N: 0.0020% or more and 0.012% or less, and B: 0.0003% or more and 0.0030% or less with a rolling reduction of 30% or more and 80% or less and then by using box annealing. The steel sheet disclosed by JP '356 is described as having excellent formability after heat treatment (quenching) has been performed and excellent toughness even after quenching treatment has been performed so that tempering can be omitted.
Japanese Unexamined Patent Application Publication No. 2000-144319 discloses a method for manufacturing a thin steel sheet having excellent formability and hardenability by hot-rolling a steel slab having a chemical composition containing, by mass %, C: 0.05% or more and 0.20% or less, Si: 0.1% or less, Mn: 0.8% or more and 2.0 or less, P: 0.02% or less, S: 0.02% or less, N: 0.005% or less, B: 0.0003% or more and 0.004% or less, Al: 0.01% or more and 0.10%, in which the relationship sol.Al (%)≧9.6×N (%) is satisfied, and in which the relationship Ti (%)≦3.4×N (%) is satisfied, and then coiling the resulting hot-rolled steel sheet at a coiling temperature of 600° C. or higher. The method disclosed by JP '319 is described as being capable of providing a steel sheet having sufficient formability to be applied to forming including press forming and being able to easily achieve high strength through quenching treatment after forming has been performed.
However, the problem that a steel sheet must have not only sufficient cold formability for severe cold forming but also excellent hardenability is left unsolved by JP '345, JP '356 and JP '319, because severe cold forming including fine blanking and cold forging is not taken into account in those publications, and because the steel sheets described above tend to become too hard and decrease in cold formability, as the ferrite grain of the steel sheets tends to become excessively fine and the pearlite fraction tends to become excessively large. Moreover, when the steel sheets described above undergo cold forming including fine blanking, there is an increase in frequency of maintenance including die replacement due to increased wear of dies, which results in an increase in the manufacturing cost of parts. Furthermore, an increase in hardenability tends to be accompanied by a decrease in the uniformity of the hardness in the width direction of the steel sheet due to an excessive hardening at the edges of the steel sheet, which results in a decrease in steel sheet yield.
It could therefore be helpful to provide a hot-rolled steel sheet having not only high excellent cold formability, but also excellent hardenability and a method for manufacturing the steel sheet. Moreover, it could be helpful to provide a hot-rolled steel sheet having not only uniform strength throughout almost the entire region including edges in the width direction, but also excellent cold formability and hardenability.
Herein, ‘a hot-rolled steel sheet’ refers to a thin steel sheet having a thickness of 2.0 mm or more and 9.0 mm or less. Moreover, herein, ‘a steel sheet has excellent cold formability’ will refer to a steel material (steel sheet) that has a Rockwell hardness of 80 or less in terms of HRB or a tensile strength (TS) of 500 MPa or less before cold forming has been performed.
Herein, ‘a steel sheet has excellent hardenability’ refers to a steel sheet that has a Vickers hardness of 420 HV or more (in the case of induction quenching) or of 350 HV or more (in the case of controlled atmospheric quenching). Moreover, herein, ‘strength is uniform throughout almost the entire region including both edges in the width direction’ refers to the case where the difference between maximum and minimum tensile strength is 60 MPa or less throughout almost the entire region including both edges in the width direction of a steel sheet (the region that is on the inner side 5 mm from the edges).
Herein, ‘controlled atmospheric quenching’ refers to a quenching method including oil-quenching after a steel sheet has been heated in an atmosphere in which carbon potential is controlled.